Semiconductor switching elements are installed in a power converter, such as an inverter, for driving inductive loads, such as motors. In order to bypass a current flowing in a semiconductor switching element when the semiconductor switching element is turned off, a diode is connected in antiparallel to the semiconductor switching element. Specifically, when the semiconductor switching element is turned off, a current, such as a flywheel current, flowing in the semiconductor switching element continues to flow through the diode.
Normal PN junction diodes have a reverse recovery time between instantaneous switching from a forward bias (conducting state) to a reverse bias (non-conducting state). During the reverse recovery time, a reverse recovery current flows through a PN junction diode due to movement of minority carriers (a few charge carriers) across the p-n junction.
For this reason, when a PN junction diode is used to be connected in antiparallel to such a semiconductor switching element, during the reverse recovery time, the reverse recovery current flows through the diode, resulting in increasing switching loss and noise.
In order to reduce the reverse recovery current, a fast recovery diode (FRD) having the minority carriers each with a reduced lifetime or a Schottky barrier diode having a very small reverse recovery time can be used as a diode connected in antiparallel to a semiconductor switching element installed in such a power converter.
On the other hand, when a MOSFET is used as a semiconductor switching element to be installed in a power converter, because MOSFETs each have an intrinsic diode intrinsically formed in antiparallel thereto, the intrinsic diode is used to cause a flywheel current to continuously flow therethrough.
Specifically, when a MOSFET is used as a semiconductor switching element to be installed in a power converter and the intrinsic diode is used to cause a flywheel current to continuously flow therethrough, during the reverse recovery time, the reverse recovery current flows through the intrinsic diode. This results in increasing switching loss and noise.
Particularly, the higher the breakdown voltage of a MOSFET is, the higher the reverse recovery current flowing trough the intrinsic diode of the MOSFET is. A MOSFET with a super junction structure (SJ) can be used as a semiconductor switching element to be installed in a power converter because it provides both a high breakdown voltage and a low resistance. However, when a MOSFET with the super junction structure (SJ) is used as a semiconductor switching element to be installed in a power converter, the reverse recovery current flowing through the intrinsic diode becomes high and has a steep time characteristic. This results in increasing loss and noise.
In order to reduce loss and/or noise due to the reverse recovery current flowing though the intrinsic diode of a MOSFET, nonpatent document 1 “POWER-MOSFET APPLICATION TECHNOLOGY”, published by Nikkan Kogyo Shimbun Ltd., discloses, on page 139, that a diode with a low breakdown voltage is connected in anti-series to the intrinsic diode of a MOSFET. This prevents a forward current from flowing through the intrinsic diode.
In order to reduce loss and/or noise due to the reverse recovery current flowing though the intrinsic diode of a MOSFET, U.S. Pat. No. 6,058,037 corresponding to Japanese Patent Application Publication No. H10-327585 discloses an external circuit that applies a low reverse voltage to the intrinsic diode of a MOSFET. The low reverse voltage applied to the intrinsic diode causes reverse recovery of the intrinsic diode, thus reducing loss due to the reverse recovery current flowing though the intrinsic diode of the MOSFET.
European Patent Application Publication No. EP1814216 corresponding to Japanese Patent Application Publications No. 2006-141167 and No. 2006-141168 also discloses such an external circuit that applies a low reverse voltage to the intrinsic diode of a MOSFET.
However, the approach disclosed in the nonpatent document 1 may complicate the circuit structure consisting of the MOSFET and the diode. The approach disclosed in the nonpatent document 1 also may create loss due to the forward voltage drop across the anti-series diode upon the MOSFET being on.
The approach disclosed in each of the US Patent and EP Patent Application Publication requires, as the external circuit, a low-voltage source for creating the low voltage, a switch connected between the low-voltage source and the intrinsic diode, and a driver for turning on the switch to thereby apply the low voltage to the intrinsic diode.
Thus, the approach disclosed in each of the US Patent and EP Patent Application Publication may complicate the circuit structure consisting of the MOSFET and the external circuit, and remains loss due to the reverse recovery current based on the low voltage.